1. Technical Field
The present invention relates to a fixing device, a fixing device control method, and an image forming apparatus, and more particularly, to a fixing device for fixing an image in place on a recording medium, and an image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those features.
2. Background Art
In image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by transferring ink or toner onto a recording sheet such as a sheet of paper. The transferred, unfixed toner image may be subsequently subjected to a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium with heat and pressure.
Various types of fixing devices are employed in electrophotographic printers, some of which includes a fixing assembly formed of a pair of opposed rotary members, such as endless looped belts or cylindrical rollers.
For example, a roller-based fixing assembly comprises an internally heated roller paired with a parallel, opposed roller that presses against and co-rotates with the internally heated roller. On the other hand, a belt-based fixing assembly comprises a thermally conductive endless belt looped around multiple rollers, at least one of which is heated to conduct heat to the fuser belt, and another is paired with a parallel, opposed pressure roller. Of the two types of fixing assembly, the belt-based configuration is advantaged over its counterpart in terms of thermal efficiency, owing to the use of the thermally conductive belt which can be immediately heated to a desired operational temperature upon startup.
One important factor that determines imaging quality of a fixing device is the strength of adhesion with which a fixed toner image adheres to a recording medium, i.e., the interfacial bonding strength between the toner layer and the printed surface of the recording medium. Since electrophotographic fixing proceeds where toner fuses and penetrates into a substrate with heat and pressure, the toner adhesion strength is influenced by various operational parameters with which a fixing device is operated to fix a toner image onto a recording medium. Examples of such parameters include a heating temperature at which the toner image is heated, a pressure force applied to the recording medium during fixing, and a conveyance speed with which the recording medium is conveyed under heat and pressure.
In addition to those operational parameters, the toner adhesion strength is also influenced by various properties of a recording medium. Various types of recording media are commercially available for printing purposes, each of which has a specific thickness and surface texture, including a wide variety of paper products ranging from normal copy paper to expensive, specially coated bond paper, as well as resin-based material such as transparency film for use in overhead projectors. Accommodating different types of recording media in a fixing device, however, would result in unstable fixing performance due to variations in toner adhesion strength caused by variations in properties between the respective recording media.
Various control methods have been proposed to stabilize performance of a fixing process regardless of the type of recording medium in use.
One such method employs an optical sensor to measure the thickness of recording medium accommodated in a fixing device, and adjusts the heating temperature of the fixing process depending on the measured media thickness to control fixing performance.
Another method allows a user to specify the type of recording medium, such as whether it is smooth copy paper or rough paper, and adjusts the amount of heat and pressure applied to the recording medium depending on the user-specified media type, thereby stabilizing fixing performance
Still another method adjusts an operational parameter using more specific, physical properties of a recording medium, such as surface smoothness, so as to obtain more reliable fixing performance than is possible with adjustment based on measured media thickness or user-specified media type.
Although effective for their intended purposes, the control methods described above have several drawbacks.
One drawback is the difficulty in acquiring sufficient property information of a recording medium, in particular, surface smoothness, which is precise and accurate enough to allow for effective control of the fixing process. Another drawback is that specifying various properties of a recording medium upon each replacement or renewal is burdensome and error-prone to a human operator, which can result in a significant failure of the fixing process due to a lack of correct, complete information of the recording medium in use.
Moreover, even where each specific piece of property information of a recording medium is properly provided, some variations in toner adhesion strength are occasionally inevitable. Such inevitability is due primarily, if not exclusively, to the fact that, in addition to the property information derived through detection or from user specification, there are several undefined or omitted factors, such as microscopic fiber structure of the recording medium and dispersion of toner over the printed surface, that need to be addressed to obtain good fixing performance.
To alleviate these drawbacks, one possible approach is to provide an allowance or extra amount of heat to be applied to the recording medium to prevent insufficient fixing of toner. Such an approach, however, can be unsatisfactory where the recording medium in use is extremely thick and requires more heat to process a toner image properly, resulting in cold offset of toner. Also, extra heat application can result in hot offset of toner as well as waste of power lost where an excessive, unnecessary amount of heat is applied to a thin recording medium that can be processed at a relatively low temperature.
In a further attempt to address the problem, a control method has been proposed for controlling operation in a thermal head printer. According to this method, the fixing device is provided with a mechanical stylus or probe which swingably moves over the surface of a recording medium before entering the fixing process. Operation of the fixing device is controlled based on the surface morphology of the recording medium, which is determined by piezoelectrically detecting vibrations of the swingable probe.